It is known from GB 1031583 to provide a screw with an elastomeric `O` ring assembled thereto and located in a recess or groove extending around the shank of the screw under the screw head, so that when the screw is installed the head may be sealed. In this patent the screw may pass through a clearance hole in a sheet metal cabinet to engage a complimentary screw thread in a component inside the cabinet. Deformation of the `O` ring by controlling the axial displacement, that is tightening the screw selectively, causes the material of the ring to flow into the clearance, and the ring is also compressed between the cabinet and the head. This provides the seal and perhaps aids security because of the frictional grip of the `O` ring.
It is also well known to provide various machine or like components with `O` rings located in peripheral grooves therein to seal against internal shoulders in a matching component. GB 2070712 is one example on the plug of a fluid valve.
In much of the prior art relating to `O` rings there is a screw which can be selectively and controllably tightened to cause the `O` ring deformation to the desired extent. In all of the other examples of prior art known to the Applicant the `O` ring is fitted to a shaft or like extending into a pre-formed hole as a so called piston fit and all of the parts are of known dimensions so that `O` ring compression and hence sealing can be predicted and calculated.
In the manufacture of thermoplastics moulded components it is often required to provide a screw thread to receive a fixing screw. The plastics material is often incapable of carrying the thread, as least with adequate mechanical strength, and the conventional solution is to provide a metal insert which has the screw thread provided in the bore and instal this insert in a pre-formed hole in the plastic by various methods. The insert has fins, ribs, teeth or the like on its exterior to provide interstices which key with the plastic and become filled by the plastic so as to fix the insert in the component. Usually the pre-formed hole in the plastic is blind and there is then no sealing problem.
However, modern plastics technology extends the use of plastics for an increasingly wide range of components, and now calls for screw threads in relatively thin sections of plastic. Conventional `O` ring screws could provide a seal between the screw and insert if used in such component but not between the insert and plastic because the insert is not selectively axially displaced and is in fact usually inserted with a fixed displacement, for example until the insert bottoms in a hole or until the top of the insert becomes flush with the component around the hole. Attempts to provide an `O` ring on the exterior of the insert in a similar place to that on a screw seem doomed to failure for this first reason of lack of selective axial movement and also secondly because the insert installation technique involves either cold pressing, which might shear the `O` ring, or softening of the plastic for example by a heated insert or use of high frequency vibrations which would displace the plastic away from the `O` ring leaving it in undeformed and unsealing condition.
The only known solution for this plastic moulding involving `O` rings is to provide a through bore with a shoulder, that is a reduced diameter portion at the bottom of the hole, place an `O` ring into the hole to rest on the shoulder, then instal the insert by the conventional techniques. The axial displacement of the insert into the component compresses the `O` ring between the insert and the shoulder to a known extent in order to provide the seal. But this technique is considered unsatisfactory. Many inserts are only a few millimeters in diameter and hence a relatively tiny `O` ring has to be placed in the component and it is easy for it to be overlooked and omitted, or for it to be misplaced so that it never goes into proper sealing position. Furthermore, if a component is of the order of 7 or 10 mm thick, and a shouldered hole is to be provided, it is inevitable that the reduced diameter hole portion will be of very small axial thickness--otherwise there is insufficient of the hole used for anchorage of the insert. So the thin flange left by the shoulder can easily deform and the result is that the `O` ring is not placed under the designed compressive load and again is not in sealing condition.
Consequently, alternative means for sealing in such situations are often used including the provision of a gasket between the component and the screw so as to overlie the end of the insert and the component, or the use of sealing compound applied. These solutions also have disadvantages, and the object of the present invention is to solve the problem and provide a new and improved solution.